A fuel cell is an electrochemical device that converts fuel and oxidant directly into electricity and a reaction by-product of water through an electrolytic reaction that strips hydrogen molecules of their electrons and protons. Ultimately, the stripped electrons are collected into some form of usable electric current, by resistance or by some other suitable means. The protons react with oxygen to form water as a reaction by-product.
Typically, a single Proton Exchange Membrane (PEM) fuel cell, operating at a temperature of around 70-80° C., consists of a combined solid polymer membrane electrolyte and two thin layers of catalysts on each side of the electrolyte, commonly called a membrane electrode assembly (MEA), which is sandwiched between two electrically conductive flow field plates (or separator plates). Generally, a single cell produces about 0.6-0.8 volt. In order to generate a higher voltage to meet practical power demands, multiple cells are commonly stacked in series to form a structure known as a fuel cell stack.
As schematically illustrated in FIG. 1, a fuel cell stack 10 is formed by compressively stacking multiple fuel cells between two endplates. Gas reactants, hydrogen or hydrogen-rich fuel 100 and oxygen or oxygen-containing air 200, are directed into the fuel cell stack through gas manifolds 110 and 210 and distributed into the anode side 120 and the cathode side 220 of individual cells. The depleted reactants are collected into the outlet manifolds 130 and 230 and then flow out the stack as indicated by streams 140 and 240. Although not shown, there may be flow passages for flowing stack coolant.
Operation of such conventional fuel cell stacks is well known for those familiar with the art, and it is well known that the performance of such conventional fuel cell stacks depends on various factors. Among many others are the flow distribution, stoichiometry, and by-product water removal from the cells. Firstly, it has been well documented in the field that the performance of a fuel cell stack having a plurality of cells is generally lower than the performance of a single cell. The decline in the performance, which is more severe for the cells located at either ends or at both ends of the stack, is believed to be due, at least in part, to the fact that reactant distribution into individual cells of a stack becomes non-uniform. The non-uniformity in reactant gas distribution becomes more pronounced for a longer stack with a larger number of cells than a short stack with a smaller number of cells. Use of longer stacks formed by multiple cells, in most cases, is usually necessary because of the requirement for meeting the power output demand.
The stoichiometry, which is defined as the ratio of the amount of the reactant gas supplied into the stack to the amount of the reactant gas consumed in the stack, is an important operation parameter as it is the inverse of the reactant gas utilization and hence directly affects the fuel cell system efficiency. In practice, the stoichiometry has an impact on limiting current density because it directly affects the mass transfer from the bulk gas channels to the interface of catalysts. More importantly, the stoichiometry plays a significant role in water management, more specifically, the water removal from the cell where it is produced. A higher stoichiometry, usually as high as 1.5 for fuel side, and 3 to 4 for air side, will enhance the mass transfer and water removal. The practice of applying high stoichiometry is particularly common and important for the cathode air as, firstly, the cathode activation is low, and secondly, the water is produced and accumulated along the cathode flow field. Operation of a fuel cell stack with high air stoichiometry, on the other hand, has been one of the biggest parasitic power consumptions due to cathode air compression and delivery.
Therefore, there is a need to reduce stoichiometry of fuel cell stacks without reducing the performance of the stack.